Subscriber communication units, including telephone instruments and other telephonic devices, cellular radiotelephones, mobile radios, and the like, often link a human operator or user with a communication network. The user may converse with others through the communication network. These subscriber units use various techniques for presenting information to users concerning the state of communication services or of the subscriber unit. For example, dial tones, ringing sounds, and busy signals are common audible annunciations that a subscriber unit presents to a user. Many subscriber units include indicator lights and displays to visually present annunciations describing dialled telephone numbers, active telephone lines in multiple line units, service conditions, and the like.
Various signalling practices that have evolved from land-line communication systems are undesirable in connection with the presentation of annunciations in radio communication systems. For example, many linguistic or voice messages and audio sounds that are presented to a user through a subscriber unit are generated at a central switching office and transmitted to the subscriber unit for presentation to the user. Such messages and sounds consume a greater amount of the electromagnetic spectrum than may be justified for the quantum of information being conveyed. As more and more calls are communicated using a finite amount of the electromagnetic spectrum, the need to utilize the spectrum as efficiently as possible becomes acute.
Moreover, conventional signalling practices do not adequately account for the fact that some of the people using a communication system speak foreign languages. Subscribers who roam in foreign-speaking areas often have difficulty understanding voice messages which are not voiced in a language with which they are conversant. This problem becomes especially troublesome in communication systems which span areas of coverage within which many languages are spoken. And, the troublesome nature of this problem is multiplied for radio communication systems which tend to have numerous diverse system states about which explanations need to be communicated to subscribers.
Still further, conventional practices are poorly suited for various signalling annunciations which must be presented to users of digital communication systems. In digital communication systems, voice data are often encoded and/or compressed for transmission through a communication network. When a circuit between two parties has been established and a call is ongoing, patching into the call to deliver additional signalling information, such as a call waiting "click", is a complicated matter due to the encoding. One possible technique for patching into an ongoing call might be to route the ongoing call to a central location, decode the digitized call data, mix the decoded call data with signalling data, re-encode the mixed call and signalling data, and route the re-encoded mixture of call and signalling data to the call's destination. However, this technique is highly undesirable due to the significant additional communication resources needed. The allocation of significant additional resources to convey simple signalling information can waste scarce electromagnetic spectrum and other network resources.